Plans for Emittance Measurement in SLAC End Station A by rockandrolldreams


									    Plans for Emittance Measurement in SLAC End
                      Station A.

1      Beamline Overview
The ESA beamline layout is shown in Figure 1, as it was for the E158 experiment. The
new ESA experimental area is the portion of beamline downstream of ’IV40’.
    The beam parameters have been evaluated at the last bend (B25-26) using ELEGANT
to track the beam from the linac exit. This evaluates the effect of SR on horizontal emit-
tance growth, which is significant. After the last bend four quadrupoles exist to provide
beam focusing for the different experiments (Q27, Q28, Q30, Q38). The use of Q38 may
interfere with beam trajectory determination from BPM 31-32, so this quadrupole has
been omitted from optics solutions for the ESA experiments.
    Q27, Q28, Q30 are roughly 100m, 90m and 50m respectively from the planned posi-
tion of the first wire scanner ’3WS1’ in the new ESA area (a second wire scanner will be
installed as part of the energy spectrometer BPM experiment).

2      Emittance Measurement Plan
Use the quadrupole scan method with Q30 to vary the beamsize and 3WS1 to measure
the beamsize.
    Use (arbitrarily) the optics configuration for the collimator wakefield experiment, and
vary only Q30 to obtain the necessary data. The emittance will then be extracted from
series of quadrupole K values and corresponding beamsizes in x and y, using the first-
order transport theory and the least-squares method, as in [1].
   The emittance calculation is simplest if only one quadrupole is varied. Multiple
quadrupoles may be used to obtain a wider range of beamsizes.

3      Measurement Issues

3.1     Data Required

Data required for each value of current in the quarupole are:

      • Quadrupole strength K, or gradient dB/dr

                              10.1 A-Line Overview

                                                                                                               Synchrotron light monitor
                                                        PR-10                                      Flip coil                                       PR-20
                               dump                             Q-11                                                       Q-19

                                                  BPM-10                        BPM-12                            BPM-17                                                             BPM-24
                               D-10 Q-10       FV-10                   B11-12            B13-14            B15-16             SL-19 SL-10     Q-20              B21-22 ST-22 B23-24


                                                                                                                                                                               End of BSY
                                                                                                                 PR-28                                              PR-33
                                                                                                                           ST-29                                             ST-29

                                                                                                           BPM-28                                          BPM-31

                                C-24           B25-26      IV-26       Q-27 SQ-27.5 Q-28             FV-28                                           Q-30                     C-37   Q-38

    Figure 1: ESA schematic
                                        Roller Screen 1                                                                  3PR-3             3PR-4

                                                                                          Target        3V-43
                                                                3BPM1    3BPM2

                                       IV-40                                    IV-41                           3Q1 3Q2       3Q3 3Q4
                                                                                                  3V-42                                                                     Beam Dump East


                                                                                                                                                                        April 11, 2002
                                                                                                                                                                        10.1, Page 1 of 1
      • Quadrupole magnetic length, not physical length (what error might this make?)
      • Beamsize in x and y

   In addition, require accurate measurement of distance from Q30 to WS1 (been guess-
ing from unscaled diagram at moment).
      Errors on above parameters?

3.2               Range of Beamsizes Required

The following range of beamsizes at the wire scanner WS1 can be obtained by varying
Q30 between zero and maximum strength.

      • σx : min 0.9 mm, max 2.2 mm
      • σy : min 0.09 mm, max 1.0 mm

    The beamsize at the wire scanner as a function of quadrupole strength K can be seen
in figure 2

                                  1                                                                                           2.2
       vertical beam size (mm)

                                                                                                  horizontal beam size (mm)

                                 0.9                                                                                           2
                                 0.8                                                                                          1.8

                                 0.7                                                                                          1.6
                                 0.3                                                                                          0.6
                                 0.2                                                                                          0.4
                                 0.1                                                                                          0.2
                                  0                                                                                            0
                                       -0.08 -0.07 -0.06 -0.05 -0.04 -0.03 -0.02 -0.01        0                                     -0.08 -0.07 -0.06 -0.05 -0.04 -0.03 -0.02 -0.01        0
                                                                                         -2                                                                                           -2
                                                                              KQ30 (m )                                                                                    KQ30 (m )

Figure 2: Vertical and horizontal beamsize at wire scanner WS1 vs Q30 quadrupole

    Since the wirescanner beamsize measurement error is < 5 µm for 100 µm beamsize
[2], this range of beamsize variation should be sufficent to extract emittance.

3.3               Residual Dispersion

If can limit horizontal dispersion to ∼30mm at 3WS1, and beam energy spread is 0.2%,
dispersion contributes 60um to beamsize (∼6% on wakefield expt x-beamsize at nominal
   Calculated effect of adding 60um to quad scan beamsize measurements and feeding
through emittance calculation, result is 10% overestimate of x-emittance.

3.4     Dipole Kicks from Quadrupoles

Some small apertures (collimators 3C1, 3C2) in ESA line downstream of Q30 which need
consideration in case of dipole kicks when scanning Q30, particularly to max strength.

                Collimator   Distance from Q30(m)   Aperture Radius (mm)
                   3C1                38.1                  19.1
                   3C2                ∼66                    8.1

   3C2 is smallest aperture and furthest from Q30. Calculate effect of dipole kick from
normal incident beam offset on Q30 (at max strength).

                offset at Q30(mm)   KQ30(m-1)   offset at 3C2(mm)    size at 3C2(mm)
      y plane            0.5         -0.0874            -5.0                 1.3
      x plane            0.3         -0.0874            4.0                  2.8

    In x-plane beamsize quite large (∼3 mm) at 3C2, needs tight alignment at Q30 to avoid
striking collimator.

 [1] Ebihara, K et al. Nuclear Instruments and Methods in Physics Research 202, p 403.
 [2] Ross, M. SLAC-PUB-5556.


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